Natural gas hydrates (NGH or clathrate hydrates of natural gases) form when water and certain gas molecules are brought together under suitable conditions of relatively high pressure and low temperature. Under these conditions, the ‘host’ water molecules will form a cage or lattice structure capturing a ‘guest’ gas molecule inside. Large quantities of gas are closely packed together by this mechanism. For example, a cubic meter of methane hydrate contains 0.8 cubic meters of water and typically 164 but up to 172 cubic meters of methane gas. While the most common naturally occurring clathrate on earth is methane hydrate, other gases also form hydrates including hydrocarbon gases such as ethane and propane as well as non-hydrocarbon gases such as carbon dioxide (CO2) and hydrogen sulfide (H2S).
NGH occur naturally and are widely found in sediments associated with deep permafrost in Arctic environments and continental margins at water depths generally greater than 500 meters (1600 feet) at mid to low latitudes and greater than 150-200 meters (500-650 feet) at high latitudes. The thickness of the hydrate stability zone varies with temperature, pressure, composition of the hydrate-forming gas, underlying geologic conditions, water depth, and other factors.
World estimates of the natural gas potential of methane hydrate approach 700,000 trillion cubic feet—a staggeringly large figure compared to the 5,500 trillion cubic feet that make up the world's currently proven gas reserves.
Most of the methane hydrate research to date has focused on basic research as well as detection and characterization of hydrate deposits. Extraction methods that are commercially viable and environmentally acceptable are still at an early stage. Developing a safe and cost effective method of producing methane hydrates remains a significant technical and economic challenge for the development of hydrate deposits.
A growing body of work indicates that when a hydrate reservoir is produced, dissociation fronts will form on both the bottom and top of the hydrate layer. The appearance of a dissociation front on the bottom of the hydrate layer is because the deeper parts of the earth are typically hotter than the shallower parts. Hydrate dissociation is a strongly endothermic process (i.e., the hydrate must draw in heat from the surrounding environment). Further, the earth below the hydrate reservoir has its heat continuously provided and replaced by even hotter layers below; thus providing an essentially endless supply of new heat to the hydrate reservoir.
The appearance of a dissociation front on the top of the hydrate layer is a less obvious phenomenon because the geothermal temperature is typically cooler than that of the hydrate layer, but given the strongly endothermic nature of hydrate dissociation it becomes evident that even heat from the earth above the hydrate layer will be drawn into the hydrate reservoir. The key difference is that the shallow earth above the hydrate layer is measurably cooler than the deep earth below the hydrate reservoir. In addition, the shallow earth above the hydrate layer (whether deep ocean floor sediments or arctic permafrost) is being continuously cooled from above. Any heat, once it has been pulled into the hydrate layer below, will not easily be replaced.
It is worth noting that the dissociation fronts on both the top and bottom of the hydrate layers are nearly horizontal and quickly move out to great radial distances from the wellbore. After the initial dissociation phase when the dissociation fronts are established, the disassociation fronts then slowly work their way towards each other, eventually meeting somewhere in the middle of the hydrate deposit, at which point the hydrate reservoir will be completely dissociated.
Produced gas in any reservoir will rise up due to its natural buoyancy. Produced gas from hydrate dissociation will tend to flow upwards and pool at the top of the hydrate reservoir. The relative initial coolness and lack of replacement heat from the shallow earth above the hydrate reservoir results in a condition whereby the ‘head space’ gas is very cool and easily reconverts to hydrates at the slightest pressure drop.
Consequently, even small pressure drops (for example, the pressure drop associated with the necessarily relatively lower pressure at a producer well that enables gas to flow toward the wellbore) can cause sufficient consequent temperature drop due to Joule-Thompson effects that dissociated methane gas from hydrates will be caused to reform hydrates in the upper ‘head space’, particularly near the wellbore. This formation of hydrates can essentially block or restrict further production.
Left unmitigated, the only solutions have been to reduce the pressure drop (i.e. lower the production rate) to a point where hydrate reformation will not occur or add heat-generating features to the production well bore. The negative economic consequences of such techniques are self-evident.